The present invention relates to communications jamming equipment and more particularly to an autoregressive peek-through communications system which permits simultaneous jamming and reception of a transmitted signal.
Communications jamming equipment has found use in situations in which it is desirable to prevent someone else from obtaining transmitted information. Typically, the information is a voice signal sent by a transmitter over a radio band, and an attempt is made to prevent the intended receiver from receiving the message.
One such technique, called look-through jamming, begins with the jamming transmitter in the "off" position and the jamming receiver looking for the existence of a transmission. When the jamming receiver detects that a transmitter has turned on, the jamming transmitter is operated nearly continuously but with periodic interruptions. The periodic interruptions, on the order of once every several seconds, are necessary to determine if the transmitter is still operating. Once the transmitter goes away, then of course, the jamming equipment can be turned off.
Another technique is the use of peek-through jamming. In this technique, an attempt is made to simultaneously jam and demodulate the transmitted signal. The jammer transmitter is operated for a period of time and then turned off for a short interval to allow a sample of the transmitted signal to be taken. The sampling takes place at or near the Nyquist rate. Thus, in the case where the transmitted signal is a voice signal, the jammer is turned on and off at the rate of approximately 6 khz of two times the normal voice bandwidth of 3 khz. The voice signal is reconstructed by low pass filtering of the samples taken.
The block diagram of such a prior art system, capable of performing both look-through and peek-through jamming is in FIG. 1. The system comprises a receiver section 70 and a transmitter section 80. The receiver 70 consists of appropriate antenna apparatus 10, RF demodulator circuit 12, sampler 18, low pass filter 14 and detection equipment 30. The jammer transmitter 80 comprises a signal generator 51, power switch 55, an RF modulator 56, bandpass filter 58, and transmit antenna 60 which may or may not be the same physical structure as receive antenna 10. In operation, either the receiver 70 or the jammer 80 is active at any given instant in time. So when, for example, power switch 55 is closed, enabling the jammer transmitter 80 to operate, the sampler 18 will be in the open position so that a sample is not being taken. Likewise, when switch 18 is closed, indicating that a sample is being taken, the jammer transmitter 80 will be deactivated by opening power switch 55.
U.S. Pat. No. 3,739,281 to Deserno et al. is an example of a look-through jamming system. It illustrates one technique for temporarily inactivating the jammer transmitter and searching through the radio band to determine if the transmitted signal is still present.
U.S. Pat. No. 4,247,946 to Mawhinney is of general interest, as it discloses a circuit capable of accurately sampling the transmitted signal as would be done by the sampler 18, and other circuitry related to accurate signal detection as would be performed by the detection equipment 30.
One of the difficulties with existing peek-through jamming equipment is that it is desirable to minimize the jammer off time, in order to maximize the effect of making the transmission unintelligible. For most radio communications situations, this also means that high powered RF amplifiers must be switched on the order of tens of microseconds, which in turn limits the amount of jamming power which can be used.
Another problem with such systems is that an intelligent transmitter might determine that it was being jammed and synchronize itself to the jammer sampling rate so that no information is sent while the jammer is off.